The proposed project involves an electrophysiological and ultrastructural investigation of modulation of swimming speed in the pteropod mollusc Clione limacina. Two peripheral modulatory inputs will be investigated, including a serotonergic pathway that enhances muscle activity without producing a direct motor response, and a direct input from motor neurons that are not directly influenced by the swim pattern generators. The role of these inputs in ballistic escape swimming will be investigated. Specifically, this investigation will include: (1) a description of the effects of serotonin perfusion on peripheral muscle cells, (2) description of activity of identified serotonergic neurons during locomotory speed changes including escape swimming, (3) description of the role of "startle" motor neurons in escape swimming, and (4) ultrastructural description of synaptic terminals of peripheral serotonin-immunoreactive neurons and startle motor neurons. Swim acceleration is accompanied by a distinct "change-of-gears" in Clione. Both central neuronal circuitry and peripheral muscle organization are arranged to produce two-geared speed changes. Since all neuron and muscle cell types of the swimming system are accessible for microelectrode recording, it is believed that the Clione swimming system is an excellent model system for investigation of the neurobiological bases of dramatic and subtle speed changes in a locomotory system. This information should be applicable to the study of gait changes in higher animals including humans. As such, the model may be useful to test the effects of specific and general perturbations on central, peripheral and modulatory subsystems within the overall Locomotory system. This project may also provide useful information for the fields of robotics and computer-generated muscle stimulation systems designed to produce walking movements in the limbs of paraplegics.